Gas turbine engines of the type used for industrial applications may employ combustor systems designed to minimize nitrogen oxide emissions. One such combustor system, disclosed in U.S. Pat. No. 5,481,866, entitled Single Stage Premixed Constant Fuel/Air Ratio Combustor, issued to Mowill on Jan. 9, 1996, is incorporated herein by reference to the extent necessary for a full understanding of such a combustor. The '866 patent discloses a combustor having an externally cooled non-perforated combustor liner that receives all combustion air from a venturi shaped premixer. Excess air that does not enter the combustor through the premixer is ducted so as to externally cool the combustor liner, and eventually re-enters the flowpath downstream of the combustion region through dilution ports. An air valve is used to directly control the amount of air supplied to the premixer so as to minimize nitrous oxide emissions at all power settings. The air valve has the effect of indirectly controlling the amount of air routed to the dilution ports.
A problem occurs when combustors of the type disclosed in the '866 patent are used in conjunction with an engine having a compressor with a relatively high compression ratio. At low engine power settings, the air valve used to control air to the premixer is mostly closed forcing most of the compressed air through the dilution ports. Although engine power is reduced, the total volume of air being pumped by the compressor at low power or idle settings remains high, resulting in a substantial increase in dilution airflow at reduced power. However, the dilution ports are necessarily sized to provide adequate backflow margin at the lower flow, higher power settings. Thus at reduced power, higher dilution flow conditions, the dilution ports overly restrict the dilution airflow causing a larger than desired pressure drop across the combustor and a loss of engine efficiency.
One solution has been to provide a separate apparatus for varying the flow area of the dilution ports at different power settings in addition to an air valve for controlling air to the premixer. A disadvantage is that such apparatus are typically very complex, adding significantly to the total cost of the combustor system.
Another solution is disclosed in the copending U.S. patent application Ser. No. 08/966,393, filed Nov. 7, 1997 which is assigned to the assignee of this application. The '393 application discloses a combustor dilution bypass system that includes an air valve and a low pressure drop combustor bypass duct. The air valve simultaneously controls both the supply of air to the premixer, and the amount of air directed into a large bypass duct. Air entering the bypass duct is reintroduced into the gas flowpath as dilution air downstream of the primary combustion zone. At low power settings the air valve directs most of the air to the bypass duct, in effect adding dilution flow to that provided through the fixed area dilution ports, whereby the pressure drop across the combustor may be controlled at an optimal level.
Notwithstanding the amount of air being bypassed, to achieve low emission there is a need to have the fuel and air thoroughly mixed in the premix injector prior to the mixture entering the combustion chamber. Failure to mix the fuel and air results in fuel rich and/or fuel lean concentrations in the combustion chamber. These concentrations lead to local flame temperatures that depart from the optimum for the minimum production of carbon monoxide and nitrogen oxides. The eventual burning of these rich concentrations results in the generation of hot regions which produce nitrogen oxides and can damage turbine components downstream of the combustor. The lean concentrations promote incomplete combustion and production of carbon monoxide and unburned hydrocarbons. This is especially a concern where the fuel is a gas as opposed to a liquid. Because a gaseous fuel will have very low momentum when injected, the compressed air with which it needs to mix can in affect trap the gas and prevent it from mixing.
Accordingly, a need exists in a low emissions combustor for a premix fuel injector that thoroughly mixes gaseous fuel and air before injecting the mixture into the combustion chamber.